In olefin epoxidation, a feed containing an olefin and an oxygen source is contacted with a catalyst under epoxidation conditions. The olefin is reacted with oxygen to form an olefin oxide. A product mix results that contains olefin oxide and typically unreacted feed and combustion products.
The olefin oxide may be reacted with water to form a 1,2-diol, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, an alcohol, or an amine.
Olefin epoxidation catalysts comprise a silver component, usually with one or more additional elements deposited therewith, on a carrier. Carriers are typically formed of a refractory material, such as alpha-alumina. In general, higher purity alpha-alumina has been found to correlate with better performance. It has also been found for example that the presence of minor amounts of impurities in the carrier such as alkali and/or alkaline earth metals and some forms of silica can have a beneficial effect.
Carriers for olefin epoxidation catalysts can be made by different processes that result in carriers having distinct morphologies. In a first process, which is disclosed in U.S. Pat. No. 4,994,589, carrier is made by a process that produces alpha-alumina support particles having a “platelet morphology”. FIG. 1 in U.S. Pat. No. 4,994,589 is a scanning electron micrograph of alpha-alumina support particles having a platelet morphology. To produce carrier with the platelet morphology, a “fluorine recrystallizing agent is used in an amount sufficient to effect conversion of the alumina to alpha-alumina having at least one substantially flat surface.” “The “substantially flat major surface” referred to herein may be characterized by a radius of curvature of at least about twice the length of the major dimension of the surface. Preferably, the particles also have aspect ratios of at least about 4:1, the aspect ratio being the ratio of the longest or major dimension to the smallest or minor dimension.” The process forms alumina having the platelet morphology which, when viewed at high magnification such as 2000×, approximates the shapes of “small plates or wafers”. As described in U.S. Pat. No. 4,994,589, “A portion of the support particles preferably are formed as “interfused” or “interpenetrated” platelets, that is, having the appearance of platelets growing out of or passing through one another at various angles.” With regard to the quantity of platelet alumina in the carrier, “Preferably, at least about 50 percent of particles of the support having a particle size of at least 0.1 micron comprise particles having at least one substantially flat major surface.” Furthermore, “These platelet-type particles frequently have substantially angular edge portions, as contrasted with amorphous or rounded edge portions of conventional support materials, including conventional alpha-alumina supports.” In a second process, “conventional” carrier, which may be referred to herein as carrier comprising non-platelet alumina, is made without using a fluorine recrystallizing agent. As described herein, carrier comprising non-platelet alumina, which is also known as non-platelet carrier, has very few, if any, particles of alumina having at least one substantially flat major surface. As used herein, no more than 25 percent of the non-platelet carrier's alumina particles have at least one substantially flat major surface. The second process typically uses small amounts of one or more bond materials to facilitate bonding of the alumina particles to one another. The bond material may partially coat some of the alumina particles and/or may appear to accumulate between the particles thereby forming bond posts. The morphology of the carrier made by the second process impacts physical characteristics of the carrier, such as surface area, pore size distribution and particle size.
Intuitively it might also be considered that the higher the surface area of the carrier, the greater the area available for deposition of the silver and therefore the more effective the silver deposited thereon. However, this is generally found not to be the case and in modern catalysts the tendency is to use a carrier with a relatively low surface area, for example a surface area of less than 1.3 m2/g, or even less than 1 m2/g.
US 2003/0162984 A1 discloses carriers which have a surface area of at least 1 m2/g. The working examples given show improved initial selectivity and activity of epoxidation catalysts based on carriers having at least 70% of the total pore volume represented by pores with diameters in the range of from 0.2 to 10 μm.
The catalyst performance may be assessed on the basis of selectivity, activity and stability of operation. The selectivity is the fraction of the converted olefin yielding the desired olefin oxide. As the catalyst ages, the fraction of the olefin converted normally decreases with time and to maintain a constant level of olefin oxide production the temperature of the reaction is increased. However this adversely affects the selectivity of the conversion to the desired olefin oxide. In addition, the equipment used can tolerate temperatures only up to a certain level so that it is necessary to terminate the reaction when the reaction temperature would reach a level inappropriate for the reactor. Thus the longer the selectivity can be maintained at a high level and the epoxidation can be performed at an acceptably low temperature, the longer the catalyst charge can be kept in the reactor and the more product is obtained. Quite modest improvements in the maintenance of selectivity over long periods yields huge dividends in terms of process efficiency.